Hybrids

Last time I wrote we had decided on our input device for the DAC output stage and now it's time to pick the gain stage. The job of the input stage is to provide an easy connection between the DAC it's fed from (or any input for that matter), get rid of any common noises and distortions (if it's a balanced design) and hand off the musical signal to the gain stage. If you look at just about any amplifier topology it can be broken down into three elements:

1. Input stage
2. Gain stage
3. Output stage

The gain stage is where most of the actual voltage amplification happens: making small musical signals into big musical signals. The choice of device here is equally critical to that of the input and output elements. In yesterday's post we covered the four device types designers can choose from and, while all four are still available to us for the gain stage, the choice narrows down a bit more in my opinion because tubes and bipolars generally have better gain possibilities than do FETS and placing an IC op amp here would be heresy if we're focused on a discrete design. Choosing a bipolar for the gain stage makes this type of amplifier a hybrid of sorts. A more purist approach might have all FETS in the amplifier, or all tubes, or bipolars, etc. However, over the years we've found that choosing the best device for a given task yields much better results sonically. So, in our amplification stages we use FETS in the input, and bipolars for gain and the output stage - although here MOSFETS also make a great choice. Bipolars have high gain characteristics as well as the ability to form a really strong and punchy sounding output stage. The main point of this post is to show how choosing the proper device for the job, within an amplifier circuit, can many times yield great sonic results.